Unified mod_offline and mod_offline_odbc

Deleted unused functions:
- `update_table()`
- web admin callbacks

Resurrected `mod_offline_odbc`.There are 2 versions of it: 
- `mod_offline_odbc` uses a new table, multiple row inserts and stores each field in a separate column. It also supports multiple hosts;
- `mod_offline_odbc_legacy`  reproduces original behavior.